1. Field of the Invention
This invention relates to a control system for a cylinder cut-off internal combustion engine in which fuel supply to some of the cylinders is cut off and running control such as cruise control is conducted therewith for the vehicle on which the engine is mounted.
2. Description of the Related Art
In a conventional multi-cylinder internal combustion engine having a plurality of cylinders, it has been proposed improving fuel consumption by switching engine operation based on the engine load, between a full-cylinder operation in which all of the cylinders are supplied with fuel to be operative and a cut-off cylinder operation in which some of the cylinders are cut off with fuel supply to be inoperative. In this type of cylinder cut-off internal combustion engine, since shock may sometimes be generated by torque fluctuation during engine operation changeover, it has also been proposed eliminating shock by adjusting the throttle opening during a transitional period of changeover, as taught in Japanese Laid-Open Patent Application No. Hei 10 (1998)-103097, for example.
It is also known to conduct running control to control a vehicle to cruise at a desired vehicle velocity set by the operator. A similar preceding vehicle follow-up control (so-called “adaptive cruise control”) is known in which a vehicle is controlled to detect the distance between itself and a preceding vehicle using radar or the like, and to travel while maintaining a desired inter-vehicle distance between itself and the preceding vehicle. In these types of control, the vehicle velocity or inter-vehicle distance when the operator manipulates a corresponding setting switch is stored as a desired velocity or distance, and the throttle opening is adjusted via an actuator such that the vehicle runs at the stored desired velocity or the vehicle runs at a desired velocity necessary for maintaining the desired inter-vehicle distance between itself and the preceding vehicle, as disclosed, e.g., in Japanese Laid-Open Patent Application No. Hei 9 (1997)-290665.
Generally, in the cylinder cut-off engines, when a fuel cutoff condition (a condition for cutting off or discontinuing the supply of fuel) is satisfied when, for example, the throttle valve is fully closed while the cut-off cylinder operation is in progress, the supply of fuel is cut off. In a range of vehicle velocity in which running control such as the cruise control is feasible, therefore, if the fuel cutoff condition such as the case that throttle valve is fully closed is satisfied, the supply of fuel is stopped under the cut-off cylinder operation.
Aside from the above, when the engine is operated by the cut-off cylinder operation, pumping loss decreases. Here, the term “pumping loss” indicates power consumed in purging a cylinder of exhaust gas and sucking in a fresh air instead and hence, means loss in the engine. As a result, when the throttle valve is fully closed while the running control is in progress, since the engine operation is switched from the full-cylinder operation to the cut-off cylinder operation in which the engine loss (pumping loss) is decreased, the vehicle tends to decelerate not sharply, rather moderately. Unless vehicle deceleration is really required, such a moderate deceleration will be better for the operator, since the operator does not feel discomfort therefrom.
However, if the operator fully closes the throttle valve to really require deceleration when the running control is in progress, since the engine operation is switched to the cut-off cylinder operation in which vehicle deceleration is made smaller than that in the full-cylinder operation, the operator may not have the feel of sufficient deceleration as required. Specifically, when the vehicle runs a downhill, for example, since the engine loss does not reach a level enough for generating engine braking effect, it may sometimes fail to produce the deceleration as desired.